OIDC single-sign on flow

Add a way for external services to authenticate a fember and access their data. **This can also be used for a single unified login across f-club services for both volunteers and fembers.**

Example of SSO buttons:
<img width="237" height="100" alt="Image" src="https://github.com/user-attachments/assets/92ca4216-5a00-49f2-be1d-e4e66156972c" />